Update .gitignore to new `dart_tool` pub cache (#15)

Update .gitignore to new `dart_tool` pub cache
diff --git a/.gitignore b/.gitignore
index 5a472d6..dfbf1b4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,7 @@
 ### Dart template
 # Don’t commit the following directories created by pub.
 .buildlog
+.dart_tool/
 .pub/
 build/
 packages